Increment revision (from v3.15a to v3.15b) to differentiate between the nightly

Win32 development builds which I expect to resume tomorrow, from the
Win32 development builds that stopped in May of 2010.

src/sbbs3/sbbsdefs.h

@@ -51,7 +51,7 @@
 /*************/
 
 #define VERSION 	"3.15"  /* Version: Major.minor  */
-#define REVISION	'a'     /* Revision: lowercase letter */
+#define REVISION	'b'     /* Revision: lowercase letter */
 #define VERSION_NUM	(31500	 + (tolower(REVISION)-'a'))
 #define VERSION_HEX	(0x31500 + (tolower(REVISION)-'a'))